Retroreflective articles are made in a variety of forms, including sheetings such as are used on traffic signs and license plates, rigid safety reflectors mounted on motor vehicles and bikes, and patches and appliques such as are applied to garments and book bags, etc. One major use of retroreflective sheeting is in the field of highway markings and signs to improve the visibility and legibility of informational signs, traffic directions, barriers, etc. to drivers.
One common type of retroreflector employs transparent microspheres, typically with hemispheric reflectors thereon. Illustrative examples of such retroreflectors are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult).
A second common type of retroreflector employs what are referred to as cube corner retroreflective elements. Such cube corner retroreflectors typically comprise a sheet having a generally planar front surface and an array of cube corner elements protruding from the back surface. In use, the retroreflector is arranged with the front surface disposed toward the anticipated location of intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection. Illustrative examples of cube corner type retroreflectors are disclosed in U.S. Pat. Nos. 3,712,706 (Stamm), U.S. Pat. No. 4,025,159 (McGrath), U.S. Pat. No. 4,202,600 (Burke et al.), U.S. Pat. No. 4,243,618 (Van Arnam), U.S. Pat. No. 4,349,598 (White), U.S. Pat. No. 4,576,850 (Martens), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appeldorn et al.) and U.S. Pat. No. 4,895,428 (Nelson et al.). Cube corner retroreflectors have commonly been employed as safety devices on bicycles, automobiles, and other vehicles as well as on traffic signs.
Cube corner retroreflectors typically have a higher retroreflective efficiency than microsphere-based retroreflectors and are sometimes preferred for application to substrates for this reason. However, retroreflective posts, cones, barrels, safety helmets, and corrugations or rivets on truck trailer surfaces require that the sheeting bend and conform to curved substrates. The cubes of cube corner retroreflectors are typically made of resins having high glass transition temperatures so that the cubes maintain their dimensions, and thus are capable of providing bright retroreflection, upon being exposed to high temperatures or high levels of humidity over time. Such resins are typically rigid (i.e., have a high flexural modulus). Unlike microsphere-based sheetings where the microspheres are generally much higher in modulus than the binder resin in which the microspheres are embedded, the cube corner retroreflective elements of cube corner retroreflectors tend to undergo significant optically degrading deformation as the retroreflector is conformed to a non-planar substrate because the high modulus cubes are typically similar in modulus to the rest of the sheeting.
U.S. Pat. No. 3,684,348 (Rowland) discloses a retroreflective composite material which is adapted to be shaped and mounted to surfaces of various configurations. The composite material comprises a flexible body portion to which a multiplicity of minute cube corner elements are adhered. The cube corner elements have a side edge dimension of up to 25 mils (625 microns), but preferably less than 10 mils (250 microns) along the side edge.
In U.S. Pat. No. 3,992,080 (Rowland), it is noted that the cube corner elements of the retroreflective composite material disclosed in U.S. Pat. No. 3,684,348 are distorted when the material is stretched during application to a support surface, and such distortion renders the cube corner faces non-orthogonal to a degree, resulting in significant loss of brightness.
U.S. Pat. No. 4,555,161 (Rowland) discloses a retroreflective laminar sheet assembly comprising flexible base and cover sheets and an array of retroreflective film pieces seated within discrete adjacent cells formed by bonding of the base and cover sheets at selected areas. One retroreflective film piece is contained within each cell and typically is made of minute cube corner retroreflective elements. Typically, there is a gap of about 1/8 to 1/2 inch (0.3 to 1.3 cm) between the edge of each film piece and the adjacent bonding area. The retroreflective laminar sheet assembly can be formed into a collar and mounted upon a traffic cone as shown in FIG. 3 of the patent. However, it is believed that the sheet assembly is not particularly useful when mounted to non-planar substrates which have intricate shapes or very small dimensions such as truck trailer rivets and corrugations because of the shear size of the gaps and film pieces in the sheet assembly. These gaps are typically much smaller than the retroreflective film pieces, which are rigid. The gaps present areas of the sheet assembly which are not capable of retroreflecting light. It is believed that if more film pieces and gaps were provided in a given unit of area of the sheet assembly to achieve greater flexibility, retroreflective brightness would be greatly sacrificed because of the width of the gaps relative to the width of the film pieces. In other words, the gaps can likely be made only so small before bonding of the base and cover sheets in the bonding areas is not possible. In addition, the patent discloses that the base and cover sheets are flexible to provide for flexible constructions, but does not disclose conformable base and cover sheets to provide for conformable constructions. Lastly, the sheet assembly is typically difficult to manufacture because the retroreflective film pieces must generally be cut and arranged in stacks in the manufacturing process.